Various hybrid powertrain architectures are known for managing the input and output torques of various prime-movers in hybrid vehicles, most commonly internal combustion engines and electric machines. Series hybrid architectures are generally characterized by an internal combustion engine driving an electric generator which in turn provides electrical power to an electric drivetrain and to a battery pack. The internal combustion engine in a series hybrid is not directly mechanically coupled to the drivetrain. The electric generator may also operate in a motoring mode to provide a starting function to the internal combustion engine, and the electric drivetrain may recapture vehicle braking energy by also operating in a generator mode to recharge the battery pack. Parallel hybrid architectures are generally characterized by an internal combustion engine and an electric motor which both have a direct mechanical coupling to the drivetrain. The drivetrain conventionally includes a shifting transmission to provide the necessary gear ratios for wide range operation.
Electrically variable transmissions (EVT) are known which provide for continuously variable speed ratios by combining features from both series and parallel hybrid powertrain architectures. EVTs are operable with a direct mechanical path between an internal combustion engine and a final drive unit thus enabling high transmission efficiency and application of lower cost and less massive motor hardware. EVTs are also operable with engine operation mechanically independent from the final drive or in various mechanical/electrical split contributions thereby enabling high-torque continuously variable speed ratios, electrically dominated launches, regenerative braking, engine off idling, and multi-mode operation.
Generally, it is desirable to perform ratio changes in a transmission such that torque disturbances are minimized and the shifts are smooth and unobjectionable. Additionally, it is generally desirable to perform releases and applications of clutches in a manner which dissipates the least amount of energy and does not negatively impact durability of the clutches. A major factor affecting these considerations is the torque at the clutch being controlled which may vary significantly in accordance with such performance demands as acceleration and vehicle loading. In certain EVTs, shift torque reductions can be accomplished by a zero or close to zero torque condition at the clutches at the time of application or release, which condition follows substantially zero slip thereacross.
EVTs are known in which range changes are controlled through a two-clutch synchronization and release process. Therein, a first clutch associated with a currently active range is carrying torque in an applied state and a second clutch associated with a currently inactive second range is carrying no torque in a released state. Shifting from the first range to the second range is accomplished by controlling the unapplied clutch to zero slip speed and the applying the clutch thereby placing the EVT in a two clutch application state. During the two-clutch application sate the engine is directly mechanically coupled to the output. The two clutch application state is exited and the second range effected by the release of the first clutch during control of the first clutch to zero slip speed. An exemplary such EVT and synchronous shift control is disclosed in co-pending and commonly assigned U.S. patent application Ser. No. 10/686,510.
While many vehicle operating situations avail themselves to smooth torque transfer between clutches through such synchronous shift controls, there are certain situations where system constraints may result in undesirable results. For example, very aggressive accelerations and decelerations may result in engine lugging or overspeed during the two-clutch application phase. Also, ratio violations may occur wherein the EVT is operating in one range at an input speed/output speed point preferred for the other range, which situation is desirably rectified.